The block diagram of FIG. 1 depicts selected portions of a prior art drive referred to by the general reference character 10. FIG. 1 particularly illustrates those portions of the drive 10 which adapt it for recording digital data on a Compact Disc (“CD”) or DVD.
The drive 10, which is usually incorporated into a digital computer, exchanges digital data with other portions of the digital computer via a computer bus 12. For purposes of the present disclosure, the drive 10 may be understood conceptually as including a control processor 14, although drives 10 may be actually constructed in various other different ways. Responsive to commands which the drive 10 receives via the computer bus 12, the control processor 14, among other things, supervises:                1. rotation of a CD or DVD 16 received into the drive 10 indicated in FIG. 1 by an arrow 18; and        2. operation of an optical subsystem 22 indicated in FIG. 1 by an arrow 26.The optical subsystem 22 focuses light, generated by a laser diode 34, to a spot 36 that is located along a track 38 which spirals inward across the surface of the CD or DVD 16. The control processor 14 operates in fundamentally the same manner for supervising rotation of the CD or DVD 16 and operation of the optical subsystem 22 both while the drive 10 records digital data onto the CD or DVD 16, and while the drive 10 reads previously recorded data from the CD or DVD 16.        
When recording data onto the CD or DVD 16, the control processor 14 may be understood as supplying to an integrated circuit (“IC”) writing current circuit 42:                1. write control data via a writing control bus 44; and        2. serial data for recording along the spiral track 38 via lines that are included in a recorded data bus 46.In turn, the writing current circuit 42 supplies a controlled electrical current to the laser diode 34 via a current output line 48 to generate a temporally changing light beam which the optical subsystem 22 focuses at the spot 36 on the track 38. Heating of the CD or DVD 16 due to the beam of light impinging at the spot 36 alters the physical properties of the CD or DVD 16 thereby recording along the track 38 the digital data which the writing current circuit 42 receives via the recorded data bus 46.        
While recording onto the CD or DVD 16, the energy of the light beam generated by the laser diode 34 must be controlled to heat the CD or DVD 16 at the spot 36 to a precise temperature needed to change the physical properties of the CD or DVD 16. Consequently, the electrical current which the writing current circuit 42 supplies to the laser diode 34 must be precisely controlled responsive to various different recording conditions which include:                1. the physical characteristics of various different types of CDs or DVDs 16 that may be loaded into the drive 10;        2. the speed at which the CD or DVD 16 rotates; and        3. the location of the spot 36 along the spiral track 38.        
The waveform diagram of FIG. 2 depicts how electrical current supplied by the writing current circuit 42 to the laser diode 34 varies during recording of a single bit of digital data onto the CD or DVD 16. Depending upon specific recording conditions, in conventional drives 10 the electrical current which the writing current circuit 42 supplies to the laser diode 34 when recording onto a CD at 52× increases from a nominal value of approximately ten milliamperes (“ma”) at time t0 to as much as several hundred ma at time t1, a time interval of approximately one nanosecond. The maximum electrical current supplied to the laser diode 34, Ip, may be as great as 350 ma. An electrical current supplied to the laser diode 34 which increases too swiftly or overshoots excessively can destroy the CD or DVD 16.
A significant performance difference required for a writing current circuit 42 adapted for recording digital data onto a CD and a writing current circuit 42 adapted for recording digital data onto a DVD arises from the smaller size spot 36 written on DVDs. The size of the spot 36 recorded onto DVDs is approximately one-seventh ( 1/7) the size of the spot 36 recorded onto CDs. Consequently, for the same rotation speed of the CD or DVD 16, data must be written seven (7) times faster when recording onto a DVD than when recording onto a CD. Correspondingly, for the same rotation speed the interval during which the light beam heats the spot 36 while writing a single bit of digital data onto a DVD is only one-seventh ( 1/7) of the interval for writing digital data onto a CD. Therefore, for media having similar physical properties the beam of light produced by the laser diode 34 must heat a DVD seven (7) times faster than the beam of light used for recording digital data onto a CD.
The write control data the control processor 14 supplies to the writing current circuit 42 via a writing control bus 44 includes data which specifies the amount of electrical current which the writing current circuit 42 supplies to the laser diode 34 while recording digital data onto the CD or DVD 16. It is advantageous for controlling the operation of the drive 10 while recording digital data if the electrical current which the writing current circuit 42 supplies to the laser diode 34 changes as linearly as practicable responsive to write control data which specifies the amount of that current.